#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
using namespace std;
#define int long long
#define nl '\n'
#define io ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
mt19937 rng((unsigned)chrono::steady_clock::now().time_since_epoch().count());
const int mod = 1000000007, mod2 = 998244353;
// change this
const int N = 200005;
int n, arr[N], pf[N], sf[N], ans = 1e18;
signed main() {
io;
cin >> n;
for (int i=0; i<n; i++) cin >> arr[i];
for (int i=1; i<n; i++) {
// calculate prefix
pf[i] = pf[i-1];
if (arr[i-1] < arr[i]) continue;
pf[i] += arr[i-1] - arr[i] + 1;
} // pf[i]: number of operations to make 1...i strictly increasing
for (int i=n-2; i>=0; i--) {
// calculate suffix
sf[i] = sf[i+1];
if (arr[i] > arr[i+1]) continue;
sf[i] += arr[i+1] - arr[i] + 1;
} // sf[i]: number of operations to make i...n strictly decreasing
for (int i=0; i<n; i++) {
ans = min(ans, max(pf[i], sf[i]));
}
cout << ans << nl;
}
